Toroidal plasma chamber for high gas flow rate process

ABSTRACT

A plasma chamber for activating a process gas, including at least four legs forming a toroidal plasma channel, each leg having a cross-sectional area, and an outlet formed on one leg, the outlet having a greater cross-sectional area than the cross-sectional area of the other legs. The plasma chamber further includes an inlet for receiving the process gas and a plenum for introducing the process gas over a broad area of the leg opposing the outlet to reduce localized high plasma impedance and gas flow instability, wherein the leg opposing the outlet defines a plurality of holes for providing a helical gas rotation in the plasma channel.

BACKGROUND

Plasma discharges can be used to excite gases to produce activated gasescontaining ions, free radicals, atoms and molecules. Activated gases areused for numerous industrial and scientific applications includingprocessing solid materials such as semiconductor wafers, powders, andother gases. The parameters of the plasma and the conditions of theexposure of the plasma to the material being processed vary widelydepending on the application.

For example, some applications require the use of ions with low kineticenergy (i.e. a few electron volts) because the material being processedis sensitive to damage. Other applications, such as anisotropic etchingor planarized dielectric deposition, require the use of ions with highkinetic energy. Still other applications, such as reactive ion beametching, require precise control of the ion energy.

Some applications require direct exposure of the material beingprocessed to a high density plasma. One such application is generatingion-activated chemical reactions. Other such applications includeetching of and depositing material into high aspect ratio structures.Other applications require neutral activated gases containing atoms andactivated molecules while the material being processed is shielded fromthe plasma because the material is sensitive to damage caused by ions orbecause the process has high selectivity requirements.

Various plasma sources can generate plasmas in numerous ways includingDC discharge, radio frequency (RF) discharge, and microwave discharge.DC discharges are achieved by applying a potential between twoelectrodes in a gas. RF discharges are achieved either byelectrostatically or inductively coupling energy from a power supplyinto a plasma. Parallel plates are typically used for electrostaticallycoupling energy into a plasma. Induction coils are typically used forinducing current into the plasma. Microwave discharges are achieved bydirectly coupling microwave energy through a microwave passing windowinto a discharge chamber containing a gas. Microwave discharges can beused to support a wide range of discharge conditions, including highlyionized electron cyclotron resonant (ECR) plasmas.

Compared with microwave or other types of RF plasma sources, a toroidalplasma source has advantages in low electric field, low plasma chambererosion, compactness, and cost effectiveness. The toroidal plasma sourceoperates with a low electric field and inherently eliminatescurrent-terminating electrodes and the associated cathode potentialdrop. The lower plasma chamber erosion allows toroidal plasma sources tooperate at higher power densities than other types of plasma sources. Inaddition, the use of high permeability magnetic cores coupleselectromagnetic energy to plasma efficiently, allowing the toroidalplasma source to operate at relatively low RF frequencies while loweringpower supply costs. Toroidal plasma sources have been used to producechemically reactive atomic gases including fluorine, oxygen, hydrogen,nitrogen, etc. for processing semiconductor wafers, flat panel displays,and various materials.

SUMMARY

No existing toroidal plasma source can operate at NF3 flow rate of above24 standard liters per minute (slm). There are increasing demands forhigh power, high gas-flow-rate plasma sources to increase throughput inplasma processing, particularly in manufacturing of flat panel displaysand solar panels. The gas flow rates required by these applications canbe tens to hundreds slm. At such high flow rates, flow dynamics and gasflow patterns strongly affect gas-plasma interaction or dissociationrate of the process gas as well as the stability of the plasma.

Techniques have been developed to control gas flow to improve plasmastability and to increase gas-plasma interaction. However, in existingplasma source designs process gases are introduced into a plasma channeleither through a single gas injection hole or multiple holes located ina small area in the plasma channel creating high plasma impedance nearthe gas injecting point. The localized gas concentration and high flowspeeds cause flow instabilities and limits the amount of gases that canbe processed through a plasma source.

The embodiments described herein provide an apparatus and a method forreducing localized high plasma impedance and gas flow instability in aplasma channel.

The apparatus consists of a plasma chamber for use with a reactive gassource, including at least four legs forming a toroidal plasma channel,each leg having a cross-sectional area, and an outlet formed on one leg,the outlet having a greater cross-sectional area than thecross-sectional area of the other legs to accommodate increased gas flowdue to dissociation of inlet gas by the plasma. The plasma chamberfurther includes an inlet for receiving the process gas and a plenum forintroducing the process gas over a broad area along the toroidal plasmachannel to reduce localized high plasma impedance and gas flowinstability. In one embodiment, the plenum introduces the process gasalong the plasma channel leg opposing the outlet, via a plurality ofholes for providing a helical gas rotation in the plasma channel.

In one embodiment, the holes can be substantially tangential to theplasma channel inner surfaces and are angled or oriented to create ahelical gas rotation in the plasma channel. The holes can be angledbetween 30 degrees and 90 degrees relative to an axial direction of theplasma channel leg, and between 45 degrees and 90 degrees relative to adirection perpendicular to the axis of the plasma channel leg. In oneembodiment, two separate but coherent gas rotations are introducedduring gas injection to improve gas-plasma interactions and to maintainflow stability.

In one embodiment, the plasma chamber can further include at least oneignition device to initiate plasma discharge. The ignition device can belocated between the plenum and the leg opposing the outlet, recessedfrom the plasma channel through a tube section, and with a purge hole inthe tube section for assisting with ignition of the plasma.

In one embodiment, a transition angle between the vertical legs of theplasma channel and the outlet can be greater than 95 degrees. Thetransition angle can range between 100 and 180 degrees for minimizingflow turbulence.

In one embodiment, the plasma channel can be smoothed to prevent flowturbulence, pressure build-up, or interaction of plasma with walls ofthe plasma channel. The NF3 flow capability of the plasma chamber can beat least 30 slm.

A buffer for introducing a process gas to a plasma chamber can includean inlet for receiving the process gas and a plenum for introducing theprocess gas over a broad area of the plasma channel to reduce localizedhigh plasma impedance and gas flow instability in the plasma channel.The plenum can define a plurality of holes for providing a helical gasrotation in the plasma channel. The holes can be substantiallytangential to the plasma channel inner surfaces and are angled ororiented to create a helical gas rotation in the plasma channel. Theholes can be angled between 30 degrees and 90 degrees relative to theaxial direction of the plasma channel leg, and between 45 degrees and 90degrees relative to the direction perpendicular to the axis of theplasma channel leg. A method for introducing a process gas into a plasmachamber includes introducing the process gas over a broad area of aplasma channel and creating a helical gas rotation in the plasma channelto reduce localized high plasma impedance and gas flow instability inthe plasma channel. The method further includes providing at least twoseparate but coherent gas rotations during gas introduction to improvegas-plasma interactions and to maintain flow stability. The methodfurther includes outputting the gas at an outlet location having across-section area greater than the cross-sectional area of the plasmachannel to prevent flow turbulence near the outlet location.

A plasma chamber for use with a reactive gas source, including means forforming at least four legs to form a toroidal plasma channel, each leghaving a cross-sectional area and means for forming an outlet on oneleg, the outlet having a greater cross-sectional area than thecross-sectional area of the other legs. The plasma chamber furtherincludes means for receiving a process gas and means for introducing theprocess gas over a broad area of the leg opposing the outlet to reducelocalized high plasma impedance and gas flow instability, wherein theleg opposing the outlet defines a plurality of holes for providing ahelical gas rotation in the plasma channel.

The embodiments described herein provide the following advantages overthe prior art. The plasma source can generate high flow rates ofactivated gases used for etching, thin film deposition and chamberclean. The plasma source can be used to abate harmful or undesirablegases. The plasma source expands the operational capability of toroidalplasma sources thereby enabling users to achieve higher processthroughput and lower process cost. The plasma source can operate at highgas flow rates and achieve high gas excitation or dissociation rate. Theplasma source can extend the NF3 flow capability of toroidal plasmasource to 30 slm or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will beapparent from the following more particular description of theembodiments, as illustrated in the accompanying drawings in which likereference characters refer to the same parts throughout the differentviews. The drawings are not necessarily to scale, emphasis instead beingplaced upon illustrating the principles of the embodiments.

FIG. 1 is a schematic representation of a toroidal low-field plasmasource for producing activated gases;

FIG. 2 illustrates of an embodiment of a swirl gas mixing device;

FIG. 3 shows a cross-sectional view of an embodiment of a toroidalplasma chamber;

FIG. 4 shows operation data of the plasma source demonstrating itsoperation at NF3 flow rate of up to 45 slm and at pressure of 100 torr;

FIG. 5A shows a top view of a gas plenum;

FIG. 5B shows a top view of another embodiment of a gas plenum;

FIG. 5C shows a cross-sectional view of the gas plenum of FIG. 5A orFIG. 5B;

FIG. 6A shows one side of an internal gas volume of the plasma channel;

FIG. 6B shows the helical gas rotation in the plasma channel withrespect to gas flowing in a vertical direction of FIG. 6A;

FIG. 6C shows the helical gas rotation in the plasma channel withrespect to gas flowing in a horizontal direction of FIG. 6A;

FIG. 7A shows a bottom view of an embodiment of a gas outlet;

FIG. 7B shows a cross-sectional view of the gas outlet of FIG. 7A;

FIG. 8A shows the calculated pressure drop in the plasma source based ona total flow rate of 120 slm at the gas outlet;

FIG. 8B shows the gas flow speed profile in the plasma source based on atotal flow rate of 120 slm at the gas outlet;

FIG. 9A shows a cooling structure 200 for the toroidal plasma source;and

FIG. 9B shows a cross-sectional view of the cooling structure of FIG.9A.

DETAILED DESCRIPTION

FIG. 1 is a schematic representation of an embodiment of a toroidallow-field plasma source 10 for producing activated gases. The source 10includes a power transformer 12 that couples electromagnetic energy intoa plasma 14. The power transformer 12 includes a high permeabilitymagnetic core 16, a primary coil 18, and a plasma chamber 20. The plasmachamber 20 allows the plasma 14 to form a secondary circuit of thetransformer 12. The power transformer 12 can include additional magneticcores and conductor coils (not shown) that form additional primary orsecondary circuits.

The plasma chamber 20 can be formed from a metallic material such asaluminum or a refractory metal, a coated metal such as anodizedaluminum, or can be formed from a dielectric material such as quartz.One or more sides of the plasma chamber 20 can be exposed to a processchamber 22 to allow charged particles generated by the plasma 14 to bein direct contact with a material to be processed (not shown).Alternatively, the plasma chamber 20 can be located at a distance fromthe process chamber 22, allowing activated neutral gases to flow to theprocess chamber 22 while charged particles recombine during the gastransport. A sample holder 23 can be positioned in the process chamber22 to support the material to be processed. The material to be processedcan be biased relative to the potential of the plasma.

The plasma source 10 also comprises a switching power supply 50. In oneembodiment, the switching power supply 50 includes a voltage supply 24directly coupled to a switching circuit 26 containing a switchingsemiconductor device 27. The voltage supply 24 can be a line voltagesupply or a bus voltage supply. The switching semiconductor device 27can be a set of switching transistors. The switching circuit 26 can be asolid state switching circuit. An output 28 of the circuit 26 can bedirectly coupled to the primary winding 18 of the transformer 12.

The toroidal low field plasma source 10 can include a means forgenerating free charges that provides an initial ionization event thatignites a plasma in the plasma chamber 20. The initial ionization eventcan be a short, high voltage pulse that is applied to the plasmachamber. The pulse can have a voltage of approximately 500-10,000 voltsand can be approximately 0.1 to 10 microseconds long. A continuous highRF voltage of 500-10,000 volts can also be used to produce the initialionization event, and the voltage is disconnected after gas breaks down.A noble gas such as argon may be inserted into the plasma chamber 20 toreduce the voltage required to ignite a plasma. Ultraviolet radiationcan also be used to generate the free charges in the plasma chamber 20that provide the initial ionization event that ignites the plasma in theplasma chamber 20.

In one embodiment, the high voltage electric pulse is applied to anelectrode 30 positioned in the plasma chamber 20. In another embodiment,the short, high voltage electric pulse is applied directly to theprimary coil 18 to provide the initial ionization event. In anotherembodiment, the short, high voltage electric pulse is applied to anelectrode 32 that is capacitively coupled to the plasma chamber 20 by adielectric. In another embodiment, the plasma chamber 20 is exposed toultraviolet radiation emitting from an ultraviolet light source 34 thatis optically coupled to the plasma chamber 20. The ultraviolet radiationcauses the initial ionization event that ignites the plasma.

The toroidal low field plasma source 10 can also include a circuit 36for measuring electrical parameters of the primary winding 18.Electrical parameters of the primary winding 18 include the currentdriving the primary winding 18, the voltage across the primary winding18, the bus or line voltage supply generated by the voltage supply 24,the average power in the primary winding 18, and the peak power in theprimary winding 18.

In addition, the plasma source 10 can include a means for measuringrelevant electrical parameters of the plasma 14. Relevant electricalparameters of the plasma 14 include the plasma current and power. Forexample, the source 10 can include a current probe 38 positioned aroundthe plasma chamber 20 to measure the plasma current flowing in secondaryof the transformer 12. The plasma source 10 can also include an opticaldetector 40 for measuring the optical emission from the plasma 14. Inaddition, the plasma source 10 can include a power control circuit 42that accepts data from one or more of the current probe 38, the powerdetector 40, and the circuit 26 and then adjusts the power in the plasmaby adjusting the current in the primary winding 18.

In operation, a gas is bled into the plasma chamber 20 until a pressuresubstantially between 1 millitorr and 100 torr is reached. The gas cancomprise a noble gas, a reactive gas or a mixture of at least one noblegas and at least one reactive gas. The circuit 26 containing switchingsemiconductor devices supplies a current to the primary winding 18 thatinduces a potential inside the plasma chamber 20. The magnitude of theinduced potential depends on the magnetic field produced by the core andthe frequency at which the switching semiconductor devices operateaccording to Faraday's law of induction. An ionization event that formsthe plasma can be initiated in the chamber. The ionization event can bethe application of a voltage pulse to the electrode 30 in the chamber 20or to the electrode 32 that is capacitively coupled to the plasmachamber 20. The ionization event can also be the application of a highvoltage to the primary winding. Alternatively, the ionization event canbe exposing the chamber to ultraviolet radiation.

Once the gas is ionized, a plasma is formed which completes a secondarycircuit of the transformer. The electric field of the plasma can besubstantially between 1-100 V/cm. If only noble gases are present in theplasma chamber 20, the electric fields in the plasma 14 can be as low as1 volt/cm. If, however, electronegative gases are present in thechamber, the electric fields in the plasma 14 are considerably higher.

FIG. 2 illustrates an embodiment of a swirl gas mixer plate 60 accordingto the prior art. The swirl gas mixer plate 60 contains a number ofconcentric holes 62, which are aligned tangentially to the inner surfaceof the plasma channel (not shown). In operation, the swirl gas mixerplate 60 injects feed gas helically into the plasma chamber 20, creatinga spiral flow and forcing the feed gas to mix and react with the plasma14. However, the swirl gas mixer plate 60 introduces the gas at aspecified location in the plasma channel, leading to erosion at thelocation due to high impedance created by the gas.

FIG. 3 shows a cross-sectional view of an embodiment of a toroidalplasma chamber 100 for minimizing flow turbulence and flow-inducedplasma instabilities, and improving gas-plasma interactions. Thetoroidal plasma chamber 100 includes a gas inlet 110, a toroidal plasmachannel 120, and a gas outlet 130. The plasma chamber is formed withmultiple sections and with multiple dielectric breaks 136 along theplasma channel. The dielectric breaks prevent induced electric currentfrom flowing in the plasma chamber, and distributes induced electricvoltage uniformly across the multiple dielectric breaks 136 therebyreducing peak electric field in the plasma channel.

The gas inlet 110 includes a buffer or gas plenum 140 for introducinggas into the plasma channel 120 over a broad area to reduce localizedhigh plasma impedance and gas flow instability. The plasma channel 120include an upper leg 122, a lower leg 124, and two side legs 126 thatform a race-track-shaped toroidal plasma topology. A plurality of gasinjection holes 142 (better illustrated in FIGS. 5A-5C) generate twoseparate but coherent gas rotations during gas injection to improvegas-plasma interactions and to maintain flow stability. It should benoted that the gas flow path in the plasma channel 120 is smoothed(e.g., having no sharp corners) to prevent flow turbulence, pressurebuild-up, or interaction of plasma with the channel walls. In oneembodiment, the upper leg 122 includes at least one ignition device 144for providing an ionization event that forms the plasma. The ignitiondevice 144 may be recessed from the plasma channel to reduce heat fromthe plasma to the electrode or the dielectric window. There canoptionally be a purge hole 146 injecting a fraction of inlet gas intotube section 148 connecting the ignition device 144 and the plasmachannel 120 to assist with the ignition of the plasma. The purge hole146 delivers fresh inlet gas to the ignition device 144 and helps tobring charged particles generated at the ignition device 144 to theplasma channel 120. The gas outlet 130 is substantially larger than thecross-section area of the plasma channel 120 to accommodate a higheramount of gas at the gas outlet 130 due to dissociation of the processgas, and to enable a smooth transition from the toroidal plasma channelto the gas outlet 130.

FIG. 4 shows operational data of the plasma source 100 (FIG. 3)demonstrating its operation at NF3 flow rate of up to 45 slm and atpressure of 100 torr. As shown, the plasma source 100 can operate athigh gas flow rates and can achieve a high gas excitation or adissociation rate. In one embodiment, the NF3 flow capability oftoroidal plasma source 100 can be at least 30 slm or higher.

FIG. 5A and FIG. 5B show top view of two embodiments of the gas plenum140 (FIG. 3) and FIG. 5C shows a cross-sectional view of the gas plenum140. The gas plenum 140 includes a plurality of holes 142 forintroducing process gas into the plasma channel 120 (FIG. 3). The gasinjection holes 142 generate a helical gas rotation in the plasmachannel 120. The embodiment of FIG. 5A creates a symmetric rotationpattern in the two halves of the top leg of the plasma channel 120,while the embodiment of FIG. 5B creates an anti-symmetric rotationpattern. FIG. 6A shows one side of an internal gas volume of the plasmachannel 120 (FIG. 3). The holes 142 are substantially tangential to theplasma channel 120 inner surfaces and are angled or oriented to generatehelical gas rotation in the plasma channel 120. FIG. 6B shows gastrajectories viewed along the axis of a side leg 126 of the plasmachannel 120. FIG. 6C shows the gas trajectories viewed along the upperleg of plasma channel 120. The helical gas rotation forces the plasma tothe center of the plasma channel, improving plasma stability as well asreducing erosion within the plasma channel 120. The helical gas rotationalso improves interaction between the process gas and plasma. The holes142 are angled between 30 degrees and 90 degrees relative to an axialdirection of the plasma channel 120 (generally shown as A), and between45 degrees and 90 degrees relative to a perpendicular direction(generally shown as B) to the axis of the plasma channel 120. Theinjection holes 142 are spread over a broad area in the plasma channel120 to prevent localized concentration of inlet gas and high localplasma impedance. Two separate but coherent gas rotations are introducedduring gas injection to improve gas-plasma interactions and to maintainflow stability. The holes 142 are also oriented tangential to the plasmachannel surface to avoid pushing the plasma towards the surfaces of theplasma channel 120 by the inlet gas.

FIG. 7A shows a bottom view of the gas outlet 130 of the plasma channel120 (FIG. 3) and FIG. 7B shows a cross-sectional view of the gas outlet130 of the plasma channel 120. In one embodiment, the cross-section areaof the gas outlet 130 is greater than twice the cross-section area ofthe plasma channel 120 to prevent flow turbulence near the gas outlet130. In some embodiments, a transition angle 128 between the verticallegs 126 of the plasma channel 120 and the gas outlet 130 is greaterthan 95 degrees. In some embodiments, the transition angle 128 can rangebetween 100 and 180 degrees for minimizing flow turbulence.

FIG. 8A shows the calculated pressure drop in the plasma source 100(FIG. 3) based on a total flow rate of 120 slm at the gas outlet 130.FIG. 8B shows the gas flow speed profile in the plasma source 100 basedon a total flow rate of 120 slm at the gas outlet 130. It should benoted that the highest pressure drop and flow speed occur at thetransition section between the plasma channel 120 and the gas outlet 130thereby illustrating the importance of having a transition angle atbetween 100 and 180 degrees for minimizing flow turbulence.

FIG. 9A shows a cooling structure 200 for the toroidal plasma source 100(FIG. 3). FIG. 9B shows a cross-sectional view of the cooling structure200 of FIG. 9A. The cooling structure is symmetric on the two sides ofthe plasma chamber, and only one side is shown in FIG. 9A and FIG. 9B.The cooling structure 200 includes an inlet tube 202, an outlet tube204, and a plurality of channels 206. The cooling structure 200 issegmented, similar to the plasma chamber 100, to multiple sections.Individual cooling sections are mounted onto each plasma chamber sectionalong a plasma channel. Dielectric tubes connect the different coolingsections to allow a coolant such as water to flow between the coolingsections. A thermally conductive pad or grease is used for improvingthermal conduction from the plasma channel to the cooling structure. Inoperation, a coolant is forced through the channels 206 to cool thetoroidal plasma source 100. The ability to cool the plasma source 100 isbeneficial because it reduces the temperature of the plasma chamber,protecting the plasma chamber material and vacuum seals. The ability tocool also allows the plasma source to operate at high power level andhigh gas flow rate, improving process throughput and reducing processcost.

One skilled in the art will realize the invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting of theinvention described herein. Scope of the invention is thus indicated bythe appended claims, rather than by the foregoing description, and allchanges that come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

1. A plasma chamber for use with a reactive gas source, comprising: atleast four legs forming a toroidal plasma channel, each leg having across-sectional area; and an outlet formed on one leg, the outlet havinga greater cross-sectional area than the cross-sectional area of theother legs.
 2. The plasma chamber of claim 1, further comprising: aninlet for receiving the process gas; and a plenum for introducing theprocess gas over a broad area of the plasma channel to reduce localizedhigh plasma impedance and gas flow instability, wherein a plurality ofholes are distributed along the plasma channel providing a helical gasrotation in the plasma channel.
 3. The plasma chamber of claim 2,wherein the holes are substantially tangential to the plasma channelinner surfaces and are angled or oriented to create a helical gasrotation in the plasma channel.
 4. The plasma chamber of claim 2,wherein the holes are angled between 30 degrees and 90 degrees in thedirection along an axis of the plasma channel, and between 45 degreesand 90 degrees in a direction perpendicular to the axis of the plasmachannel.
 5. The plasma chamber of claim 2, further comprising at leastone ignition device along the plasma channel.
 6. The plasma chamber ofclaim 5, wherein the ignition device is recessed from a plasma channelsurface and includes a purge hole for assisting with ignition of theplasma.
 7. The plasma chamber of claim 3, wherein two separate butcoherent gas rotations are introduced during gas injection to improvegas-plasma interactions and to maintain flow stability.
 8. The plasmachamber of claim 1, wherein a transition angle between the vertical legsof the plasma channel and the outlet is greater than 95 degrees.
 9. Theplasma chamber of claim 8, wherein the transition angle can rangebetween 100 and 180 degrees for minimizing flow turbulence.
 10. Theplasma chamber of claim 1, wherein the plasma channel is smoothed toprevent flow turbulence, pressure build-up, or interaction of plasmawith walls of the plasma channel.
 11. The plasma chamber of claim 1,wherein an NF3 flow capability of the plasma chamber is at least 30 slm.12. A buffer for introducing a process gas to a plasma chamber,comprising: an inlet for receiving the process gas; and a plenum forintroducing the process gas over a broad area of the plasma channel toreduce localized high plasma impedance and gas flow instability in theplasma channel.
 13. The buffer claim 12, wherein the plenum defines aplurality of holes for providing a helical gas rotation in the plasmachannel.
 14. The buffer of claim 13, wherein the holes are substantiallytangential to the plasma channel inner surfaces and are angled ororiented to create a helical gas rotation in the plasma channel.
 15. Thebuffer of claim 13, wherein the holes are angled between 30 degrees and90 degrees in a direction along an axis of the plasma channel, andbetween 45 degrees and 90 degrees in a direction perpendicular to theaxis of the plasma channel.
 16. A method for introducing a process gasinto a plasma chamber, comprising: introducing the process gas over abroad area of a plasma channel; and creating a helical gas rotation inthe plasma channel to reduce localized high plasma impedance and gasflow instability in the plasma channel.
 17. The method of claim 16,further comprising providing at least two separate but coherent gasrotations during gas introduction to improve gas-plasma interactions andto maintain flow stability.
 18. The method of claim 16, furthercomprising outputting the gas at an outlet location having across-section area greater than the cross-sectional area of the plasmachannel to prevent flow turbulence near the outlet location.
 19. Aplasma chamber for use with a reactive gas source, comprising: means forforming at least four legs to form a toroidal plasma channel, each leghaving a cross-sectional area; and means for forming an outlet on oneleg, the outlet having a greater cross-sectional area than thecross-sectional area of the other legs.
 20. The plasma chamber of claim19, further comprising: means for receiving a the process gas; and meansfor introducing the process gas over a broad area of the leg opposingthe outlet to reduce localized high plasma impedance and gas flowinstability, wherein the leg opposing the outlet defines a plurality ofholes for providing a helical gas rotation in the plasma channel.